They began exchanging messages a couple of years ago, the girl testified.

During testimony, prosecutor Chuck Boring showed jurors the photo of Harris’ penis that was sent to the girl.

Prosecutors say Harris intentionally killed Cooper by leaving him for hours in a vehicle parked outside his workplace. Harris’ lawyers called the death accidental.

Harris moved from Tuscaloosa, Alabama to Georgia in 2012 to take a job with The Home Depot’s corporate office. He faces life in prison if convicted of murder.

Allegations that Harris had sexually explicit online conversations with an underage girl are completely unrelated to the murder and child cruelty charges he faces in the death of his son, defence attorney Maddox Kilgore has said previously.

But prosecutors contend that those alleged actions can provide evidence of a motive for the murder charges and can demonstrate Harris’ state of mind leading up to and on the day of the boy’s death.

The 17-year-old, whose identity was shielded by Judge Mary Staley Clark, was 15 years old at the time she and Harris began exchanging sexual messages, according to Monday’s testimony.

They first began communicating with one another on the anonymous service called Whisper and later on another service called Kik, the Journal-Constitution reported.

Eventually, the girl told Harris she was actually 15 years old, but that didn’t deter him from continuing their conversation, she told jurors.

“He told me he wanted me to make him a naughty old man,” she testified, adding he said he wanted to have sex with her.